I would think FM, your assumptions are correct and each building has to be treated as separate premises/workplace, also the employer, as the Responsible Person, has to implement article 8 to 22, which includes conducting a FRA for both places, which means no special forms just two FRA are required. Also as The Regulatory Reform (Fire Safety) Order 2005 is involved, it is essential.
If the second building is a multi-occupied premises, take note, a premises means any premises or parts of premises, not being domestic premises, used for the purposes of an employer's undertaking, which are made available to an employee of the employer as a place of work and including, any place within the premises to which such employee has access while at work and any room, lobby, corridor, staircase, road, or other place, used as a means of access to or egress from that place of work or where facilities are provided for use in connection with that place of work, other than a public road.